Sleeping bags come in many shapes depending upon their intended use and cost. Mummy bags, semi-rectangular bags, and fully rectangular bags are the most common types and shapes of bags utilized for outdoor camping.
Mummy bags are generally fairly snug and are shaped to contour the body of occupants. Such an arrangement keeps the heat in by decreasing the air movement within the bag.
Semi-rectangular bags, in contrast, tend to contour the upper half of the occupant's body but have a much wider leg and foot section than that found in the mummy bag configuration. While semi-rectangular bags are generally more comfortable than mummy bags, because of the extra knee and leg room, bag warmth is decreased and weight and cost increased because of the extra fabric, insulation, and labor required for fabrication.
Fully rectangular bags tend to be the widest and most comfortable in that they do not restrict movement of the occupant's body to any substantial degree. However, fully rectangular bags are the least efficient (for their weight) in colder environments. Also, generally speaking, rectangular bags are too heavy, bulky, and inefficient to be considered for cold or high-altitude outdoor backpacking or mountaineering.
The warmth and weight of a sleeping bag is determined by the bag's design, shape, whether it has a foot or not, how much insulation is enclosed, the type and quality of insulation, and the type and quality of fabric used on the inside and outside of the bag.
U.S. Pat. No. 4,888,828, issued Dec. 26, 1989, relates to a sleeping bag device for bivouac use during mountain climbing, hiking and the like. The sleeping bag device disclosed in the aforesaid patent incorporates an arrangement alleged to provide higher warmth retention efficiency. In particular, the sleeping bag device of U.S. Pat. No. 4,888,828 incorporates elastic compression means, for example, rings of threads made of rubber, to constrict the sleeping bag and reduce the cross-sectional area of the inner chamber of the sleeping bag device, the purpose being to prevent the interior of the sleeping bag from communicating with the ambient air and thus avoid escape of warm air from the inner chamber as well as to prevent the introduction of cold ambient air into the sleeping bag device.
The principal location of such constriction of the bag is at the neck of the sleeper, the elastic compression member employed for such purpose keeping the bag in resilient contact with the neck of the sleeper. That is, the arrangement of U.S. Pat. No. 4,888,828 employs at least one elastic member, normally at the neck of the sleeper, which always serves to reduce the cross section of the inner chamber at least at the neck location of the occupant. The elastic member is always under tension when the cross section of the inner chamber does not have a reduced cross section or volume.
It will be appreciated that such an arrangement fails to add to the comfort of the occupant insofar as movement of the occupant's body is concerned. The mummy bag arrangements of the type shown, for example, in the aforesaid patent do not allow much freedom of body movement, restraining movement, for example, of the knees and legs, a restriction which is a particular irritant and source of discomfort to sleeping bag users. In addition, restricting bands of the type illustrated in the aforesaid patent which are always under tension when the inner chamber is at normal volume or cross section will cause some compression of the insulation and lessen the effectiveness of the bag to retain warmth and keep out the cold when occupied.